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Measuring the Age Structure of a Population

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Age is a very important biological characteristic of an individual. United Nations defines age of an individual as “the estimated or calculated interval of time between the date of birth and the date of census, expressed in completed solar years”. Since age determines the physical capacity and mental maturity of an individual, every society uses age as an integral part of social organization. Social roles and responsibilities are assigned in accordance with an individual’s age.

The age structure of a population determines the number of persons available for different social categories. For instance, the size of labour force available in a population depends on its age structure. Planning for various social and physical amenities in a region also requires data on age distribution. In addition, age is biologically related to fertility and mortality.

The interrelations between age structure and a wide range of social, demographic, economic and political phenomena make the classification of a population by age groups especially important. The age distribution of a population provides the basis of all detailed demographic analysis. For instance, the fertility and mortality levels of two populations can meaningfully be compared only when the age structures of the populations are taken into account. Data by age distribution is indispensable for the construction of life tables and for making projections.

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Keeping in mind the significance of age structure, question on age is included in all censuses and surveys the world over. Data on age are, however, less reliable. In the underdeveloped societies, the illiterate or semi-literate persons do not know the exact age, and, therefore, at the time of counting, the enumerators generally resort to guess. Even in developed societies, there is a deliberate tendency to concentrate on certain ages, e.g., number ending 0 or 5 or some other even numbers. Age preference in varying degree can be seen in almost all the societies.

This results in the concentration of an abnormally large number of individuals on certain ages. This problem is known as age heaping of data. Errors in data on age can also occur because of carelessness in reckoning age, misunder­standing of the question or deliberate attempt to conceal the real age. Before such data are put to any analysis, the user must make necessary corrections.

Measures of Age Structure:

The age structure of a population can be analyzed in a number of ways. The most commonly used method is the one in which percentage distribution of population in various age groups is worked out. Once the percentage distribution in different age groups is available, one can compare the age structures of two populations, or examine the temporal changes in age structure of a single population. The percentage figures are sometimes plotted on a special type of bar graph, which is known as age pyramid.

In an age pyramid, various horizontal bars represent successive ages from the lowest at the bottom to the highest at the top, and the scale is shown along the horizontal axis at the bottom. In an age pyramid one can show the age structure separately for the two sexes, and for rural and urban areas.

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In addition, various measures of central tendency, viz., mean, median and mode can also used for the analysis of age structure. However, the most commonly used measure of central tendency is the median age of population. Median age of a population is the age that divides all the individuals of the population in two equal halves. Since data on age composition are normally available for various five-year age groups, the median age can be calculated using the conventional statistical formula for the grouped data (see for instance, Mahmood, 1998).

Another measure to study the age structure of a population is what is commonly known as dependency ratio. Dependency ratio of a population provides the number of dependents in the population for every 100 working persons. Here, children below 15 years and persons aged 60 years and above (or sometimes 65 years depending upon the prevailing social and economic condi­tions and quality of life) are considered as dependent on people inthe working age group, i.e., between 15 and 59 years (or 64 years).

The dependency ratio is computed in the following manner:

Where, P0-14, P60+ and p15. 59 refer to populations in the age groups 0-14 years, 60 years and above, and 15-59 years, respec­tively. In cases, where the lower limit of dependent aged is 65 years, necessary modifications are done accordingly in the numerator and denominator in the formula.

With decline in the birth and death rates the phenomenon of population ageing has attracted a widespread attention, particu­larly in the developed parts of the world, during the recent past. Population ageing is defined as the process whereby the proportion of children in the population decreases and that of old people increases. The ageing of population can be shown with the help of median age of population. Sometimes, simple percentage share of ‘dependent aged’ in the population is also used.

However, such percentages do not provide an accurate estimate of ageing as they are determined by the relative size of population in all other age brackets. Migration of population, which is highly age selective, may affect the proportion of ‘aged dependents’ in the population. Petersen (1975) has, therefore, suggested a measure, which he called as ‘index of ageing’.